1. Field of the Invention
The invention relates to a method and an apparatus for discriminating between electromagnetic interference signals that are beamed in externally and electromedical scanning signals, particularly in input signals of cardiological implants such as cardiac pacemakers, defibrillators and the like, in neurological stimulus detection, etc.
2. Background Art
The problem to be addressed by the invention is the increasing xe2x80x9cpoisoningxe2x80x9d of our environment with electromagnetic radiation from a wide variety of sources. All electronic devices, particularly cardiological implants, are exposed to numerous different environmental electromagnetic interferences (EMIs). This problem has been exacerbated by the dramatic increase in EMI signal sources, such as alarm systems, anti-theft systems, mobile telephones, etc., and the heightened sensitivity of modern electromedical devices for example, modern cardiological implants detect and assess extremely small signals in order to diagnose the actual status of the heart as accurately as possible and, based on this assessment, generate the optimum therapeutic pulses for the dysfunctional heart. Merely shielding the implant with a metal housing is inadequate, because interference signals can be radiated in via the connected signal lines and their inputs on the implant.
To remedy this situation, it is known from the prior art to provide special lead-throughs for the implant connections, which, however, are only sufficiently effective for very high frequencies, such as those used in mobile telephones. Lower frequencies, such as are generated by, for example, alarm or anti-theft systems, mains frequencies and other industrial sources of interference signals, cannot be shielded as well as higher frequencies, and represent the primary threats to the proper function of the implant.
In this connection, a further proposal is the use of bipolar supply lines and electrodes, which at least reduce the level of interferences. These types of signal lines also cannot completely eliminate interferences, however.
Finally, it is known from the prior art, for example U.S. Pat. Nos. 5,292,348, 5,817,134 or U.S. Pat. No. 5,857,977 to identify and distinguish different tachycardia through the detection of the time of the occurrence of signals at two electrode poles. Complicated evaluation algorithms such as the cross-phase spectrum or the cross-correlation of the signals are used here.
Based on the outlined problem, it is the object of the invention to provide a method of discriminating interferences between EMI interference signals and actual scanning signals, e.g., of cardiological implants, the method being especially simple to implement.
In accordance with the invention, this object is accomplished by a method of discriminating between externally-radiated EMI interference signals and electromedical scanning signals, particularly in input signals of cardiological implants, comprising the following steps:
the preamplification of the scanning signals by means of two preamplifiers;
the comparison of the preamplified scanning signals by a comparator;
the phase-sensitive evaluation of the comparator output signal for differentiating between scanning signals and EMI interference signals;
the feedback of the comparator output signal to an amplification control of the preamplifiers such that the comparator output signal is minimized when an interference signal is detected.
The foundation of the invention is the recognition that the EMI interference signals, particularly with respect to bipolar scanning electrodes that are used in cardiological implants, are far-field signals whose magnitude is a function of the spacing between the two electrodes. In telecommunications terminology, this spacing can be understood as the xe2x80x9cdipole spacingxe2x80x9d between the two electrodes.
In contrast to EMI interference signals, the scanning signal, that is, the cardiological signal detected by the bipolar electrode of a cardiac pacemaker, is a near-field signal, which is independent of the dipole spacing. The cardiological near-field signal originates from, for example, the electrical ventricular stimulus conduction successively passing the bipolar electrodes. The two electrodes of the bipolar arrangement are charged with different potentials at a specific time. The near-field signals at the two electrodes are therefore not in phase.
The invention capitalizes on the above-described difference between near and far fields. EMI interference signals from a specific source occur, for example, as two in-phase signals of different amplitudes at the two bipolar electrodes of a pacemaker. This difference in amplitudes actually generates the interference signal, which is registered as an interference by the scanning-and-evaluation circuit of an implant.
The method of the invention and the corresponding apparatus, which are described in the ensuing description of an exemplary embodiment in conjunction with the attached drawing, are provided for eliminating this problem.